A method for estimating in situ reaction rates from push-pull experiments for arbitrary solute background concentrations

This paper presents a general method and the mathematical equations for estimating in situ reaction rates from the tracer and reactive solute breakthrough curves obtained from single-well push-pull tests (PPTs). The in situ zeroth-order reaction rates can be obtained through the linear regression of the net mass transfer of reactive solutes versus time. The method was first tested for scenarios of various concentrations of reactive constituents and conservative tracer in background and injection solution using a numerical reactive transport model and was then applied to a set of field biostimulation data from PPTs performed at the U.S. Department of Energy's Natural and Accelerated Bioremediation Research Program's Field Research Center (Oak Ridge, TN). The results show that the method is general and can be applied to each of the scenarios as long as the concentrations in background water are known. While the method presents practitioners with a simplified and economic tool for a first approximation analysis of in situ reaction rates from PPT data, the derived reaction orders and rates are apparent and bulk properties by nature, masking the complexity of competing reactions regarding the reactive solute of interest.